The following abbreviations and terms are herewith defined:                3GPP third generation partnership project        CCFI control channel format indicator (alternatively Cat0)        DL: downlink        Node B base station, or evolved node B of an LTE system        E-UTRAN evolved UTRAN        FDD: frequency division duplex        FS1: frame structure 1        FS2: frame structure 2        LTE long term evolution of 3GPP        MCS modulation and coding scheme        Node B base station or similar network access node        OFDM orthogonal frequency division multiplex        TDD time division duplex        UE user equipment (e.g., mobile equipment/station)        UL uplink        UMTS universal mobile telecommunications system        UTRAN UMTS terrestrial radio access network        VOIP voice over Internet protocol IP        
3GPP is standardizing the long-term evolution (LTE) of the radio-access technology which aims to achieve reduced latency, higher user data rates, improved system capacity and coverage, and reduced cost for the operator. The current understanding of LTE relevant to these teachings may be seen at 3GPP TR 25.814 (v7.1.0, 2006-09) entitled PHYSICAL LAYER ASPECTS OF EVOLVED UTRA and herein incorporated by reference. Both FDD and TDD are considered in LTE. Due to their difference in frame structure and duplex mode, some designs for FDD and TDD can be different (see section 6.2 et seq. of TR 25.814). As stated at section 6.2.1, “E-UTRA, when operating in TDD mode-of-operation, may face additional interference scenarios, compared to when operating in FDD mode of operation. More specifically, direct UE-to-UE and BS-to-BS interference may occur both within one carrier and between neighbour carriers.” As LTE develops it is clear that there are to be some differences as between the FDD and TDD modes of operation.
Some general principles of scheduling in E-UTRAN are recited at section 7.2.1 of TR 25.814. The Node B scheduler dynamically controls which time/frequency resources, such as subframes, are allocated to a certain user at a given time. Downlink control signalling informs UE(s) what resources/subframes and respective transmission formats have been allocated. The scheduler can instantaneously choose the best multiplexing strategy from the available methods; e.g. frequency localized or frequency distributed transmission. The flexibility in selecting subframes and multiplexing UEs will influence the available scheduling performance. Scheduling is tightly integrated with link adaptation and hybrid automatic repeat request HARQ.
In FDD, it has been agreed that there are at most 3 OFDM symbols in each TTI that are reserved for control signaling. Consider for this description that a TTI is one subframe. Those 3 OFDM symbols are to include DL and UL scheduling grants as well as CCFI/Cat0 information that gives the format of the control channel. The CCFI/Cat0 is length 2 bits and indicates how many OFDM symbols are used for control (see page 12 of document RP-070271, STATUS REPORT RAN WG1 TO TSG-RAN #36; Busan, Korea; 28 May-1 June 2007). The size of the DL control resource limits the number of UEs that can be scheduled. A rough calculation shows that if 3 OFDM symbols are reserved for control in each DL subframe, there are 8DL and 7UL UEs can be scheduled per TTI in a system having a 10 msec radio frame as in LTE. However, in the TDD system, since the DL and UL configuration can be asymmetric, more limitations exist in the scheduling and so the 8DL/7UL limit may not be attainable in certain instances due to the asymmetry that may be present. This invention addresses some of those problems in TDD UL scheduling, which by the above framework must be done in conjunction with the DL scheduling.
As further background, there are currently two different frame formats in LTE, previously known as a fixed frame structure (one 10 msec radio frame=two 5 msec frames each having seven traffic timeslots or subframes) and generic (to allow backward compatibility). These are currently referred to as frame structure 1 FS1 and frame structure 2 FS2. The TDD mode of LTE may have asymmetric DL and UL subframe allocation for both FS1 and FS2. For example, there may exist a TDD FS2 with 6DL subframes and 1UL subframe being allocated in a 5 msec frame. Given the above considerations, different approaches become available to deal with the asymmetry problem. One option is to constraint the UL scheduling grant for the UL subframe in the first DL subframe. Another option is to allow the UL scheduling grant for the UL subframe in more than the first DL subframe. And that choice must also enable one to send the UL scheduling grant for a TDD FS2 with an opposite symmetry, for example scheduling 5UL subframes and 2 DL subframes. The mapping becomes quite difficult, especially considering the limits on scheduling overhead already agreed in LTE.
Two proposals to solve the UL scheduling grant problem for the asymmetric scheduling problem is in document R1-071868, entitled DOWNLINK CONTROL SIGNALLING FOR E-UTRAN TDD (3GPP TSG RAN1 LTE TDD AdHoc; Beijing, China; Apr. 17-20, 2007; by Motorola). Where there are less UL subframes than DL subframes, there is simply a one-to-one correspondence so the ith UL subframe is allocated in the ith DL subframe. One of those proposals for more UL subframes than DL subframes is to use radio resource control RRC signaling on the synchronization channel DL-SCH to schedule UEs for UL resources that do not match one-to-one with DL resources. That paper admits that this is a high price in UL scheduling grant overhead. The other of those proposals is to create additional ‘control regions’ in some DL subframes that are associated with UL subframes that do not match one-to-one with other of the DL subframes, with a map of the association sent on a broadcast channel D-BCH. This is also seen as a higher overhead than would be ideal.
Another proposal is set forth in document R1-071882, entitled TTI INDICATION FOR LTE TDD (3GPP TSG RAN1 LTE TDD AdHoc; Beijing, China; Apr. 17-20, 2007; by CATT). This proposes to use explicit TTI indications in the form of a bitmap in the control signaling format to denote the location of subframes being allocated (this paper also makes alternative proposals). As with R1-071868, this proposal is seen to be high in control overhead, and R1-071882 does not appear to address how to minimize that overhead.
What is needed in the art is a bandwidth efficient way to schedule UL and DL resources in the LTE system when the number of DL and UL subframes being scheduled by the same Node B in different frames are asymmetric in both directions.